Palaeontologia Electronica http://palaeo-electronica.org OLIGOCENE TERRESTRIAL STRATA OF NORTHWESTERN ETHIOPIA: A PRELIMINARY REPORT ON PALEOENVIRONMENTS AND PALEONTOLOGY Bonnie F. Jacobs, Neil Tabor, Mulugeta Feseha, Aaron Pan, John Kappelman, Tab Rasmussen, William Sanders, Michael Wiemann, Jeff Crabaugh, and Juan Leandro Garcia Massini ABSTRACT The Paleogene record of Afro-Arabia is represented by few fossil localities, most of which are coastal. Here we report sedimentological and paleontological data from continental Oligocene strata in northwestern Ethiopia. These have produced abundant plant fossils and unique assemblages of vertebrates, thus filling a gap in what is known of Paleogene interior Afro-Arabia. The study area is approximately 60 km west of Gondar, Chilga Woreda; covers about 100 km2; and represents as few as 1 Myr based on radiometric dates and paleomagnetic chronostratigraphy. The sedimentary strata are 150 m thick, and dominated by kaolinitic and smectitic mudstones and airfall tuff deposits. Five main paleosol types are interpreted as representing Protosols (gleyed or ferric), Histosols, Gleysols, Vertisols, and Argillisols. Varied, poor drainage conditions produced lateral variation in paleosols, and stratigraphic variation probably resulted from lateral changes in drainage conditions through time. Vertebrate fossils occur in sediments associated with ferric Protosols and occur with fruits, seeds, and leaf impressions. Plant fossils also occur as in situ forests on interfluves, leaf and flower compressions associated with in situ carbonized trees in overbank deposits (Gleyed Protosols), and compressions of leaves, twigs and seeds in tuffs. Plant fossil assem blages document diverse forests, from 20-35 m tall, of locally heterogeneous composi tion, and representing families occurring commonly (legumes) or uncommonly (palms) in forests today. Sedimentological and paleobotanical data are consistent with a nearly flat landscape where a meandering river and ample rainfall supported lush vegetation. Over time, the region was subject to intermittent ashfalls. A unique fauna of archaic mammalian endemics, such as arsinoitheres and primitive hyracoids, lived here with the earliest deinotheres. Bonnie F. Jacobs. Environmental Science Program, Southern Methodist University, P.O. Box 750395, Dal las, Texas 75275-0395, USA. [email protected] Neil Tabor. Department of Geological Sciences, Southern Methodist University, P.O. Box 750395, Dallas, Texas 75275-0395, USA. [email protected] Mulugeta Feseha. Institute of Development Research, Department of Geology and Geophysics, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia. [email protected] Aaron Pan. Department of Geological Sciences, Southern Methodist University, P.O. Box 750395, Dallas, Texas 75275-0395, USA. [email protected] Jacobs, Bonnie F., Tabor, Neil, Feseha, Mulugeta, Pan, Aaron, Kappelman, John, Rasmussen, Tab, Sanders, William, Wiemann, Michael, Crabaugh, Jeff, and Garcia Massini, Juan Leandro, 2005. Oligocene Terrestrial Strata of Northwestern Ethiopia: A Preliminary Report on Paleoenvironments and Paleontology, Palaeontologia Electronica Vol. 8, Issue 1; 25A:19p, 852KB; http://palaeo-electronica.org/paleo/2005_1/jacobs25/issue1_05.htm JACOBS ET AL.: OLIGOCENE TERRESTRIAL DEPOSITS FROM ETHIOPIA John Kappelman. Department of Anthropology, University of Texas at Austin, 1 University Station C 3200, Austin, Texas 78712, USA. [email protected] Tab Rasmussen. Department of Anthropology, Washington University, Campus Box 1114, St. Louis, Mis­ souri 63130-4899, USA. [email protected] William Sanders. Museum of Paleontology, University of Michigan, 1109 Geddes Avenue, Ann Arbor, Mich­ igan 48109, USA. [email protected] Michael Wiemann. Center for Wood Anatomy Research, USDA Forest Service, Forest Products Labora­ tory, One Gifford Pinchot Drive, Madison, Wisconsin 53726-2398, USA. [email protected] Jeff Crabaugh. Oklahoma State University, T. Boone Pickens School of Geology, 105 Noble Research Center, Stillwater, Oklahoma 74078-3031, USA. [email protected] Juan Leandro Garcia Massini. Department of Geological Sciences, Southern Methodist University, P.O. Box 750395, Dallas, Texas 75275-0395, USA. [email protected] KEY WORDS: Ethiopia; paleosols; paleobotany; vertebrate paleontology, Paleogene, Oligocene PE Article Number: 8.1.25A Copyright: Paleontological Society May 2005 Submission: 8 January 2005. Acceptance: 6 March 2005 INTRODUCTION ably rich in fossil plants and have produced unique associations of endemic archaic and derived mam­ The Paleogene fossil record is sparse com­ mals. Abundant paleosols document paleoenviron­ pared with the Neogene record for continental trop­ ments independently of the fossils. This wealth of ical Africa, despite the much longer duration of the paleontological and palaeoenvironmental data pro­ Paleogene (43 vs. 23 Ma). The abundance of Neo­ vides information that makes Chilga unique for the gene fossil localities is related to development of African Paleogene. the topographically complex East African Rift, which generated sediment accommodation space Geological Background and created ample opportunities for fossilization. The study site is located in an area of approxi­ However, limited exploration for older sites has 2 also contributed to the shortage of Paleogene mately 100 km , 60 km west of Gondar in Chilga localities. Continuing investigations of pre-rift sedi­ Woreda, Amhara Region, northwestern Ethiopia mentary deposits of eastern tropical Africa aim to (Figure 1). Geologically, the area is characterized fill this gap and have produced significant fossil by massive flood basalts as much as 2000 m thick, sites from the Eocene of Tanzania and the Oli­ emplaced approximately 30 million years ago, prior gocene of Ethiopia (Herendeen and Jacobs 2000; to Miocene rifting (Hofmann et al. 1997). Clastic Harrison et al. 2001; Gunnell et al. 2003; Kappel­ and volcaniclastic sediments occur interbedded man et al. 2003; Murray 2003; Jacobs and Her­ with volcanic deposits in a basin formed by faulting endeen 2004; Sanders et al. 2004). These new of the basalts in the middle to late Oligocene. localities provide unique opportunities to document Approximately 150 m of fossiliferous sedimentary Paleogene plant and vertebrate evolution, paleo­ strata are exposed in outcrops across the basin, ecology, and paleoclimate in interior tropical Afro- but the most complete and best-documented sec­ Arabia, when the biota was evolving in isolation tion occurs along the Guang River. In this section, from those of other continents, and experiencing radiometric dates and paleomagnetic reversal stra­ significant global climate changes. tigraphy constrain the age of about 100 m of sedi­ In this paper we assess the paleoecological ment to between 27 and 28 Ma, the limits of Chron implications of late Oligocene fossils and sedi­ C9n, or earliest late Oligocene (Feseha 2002; Kap­ ments located in Chilga Woreda (henceforth pelman et al. 2003; Cande and Kent 1995; Figure “Chilga”), west of Gondar, on Ethiopia’s northwest­ 2). ern plateau (Figure 1). These deposits are remark­ Plant and vertebrate fossils occur throughout the Chilga deposits. Faunal assemblages, domi­ 2 JACOBS ET AL.: OLIGOCENE TERRESTRIAL DEPOSITS FROM ETHIOPIA Figure 1. Map of Chilga and surrounding area modified from Kappelman et al. (2003). (a) Map of Afro-Arabia with Chilga (C; red), and Paleogene vertebrate and paleobotanical localities marked P (blue) and V (green) respectively. (b) Location of Chilga in Ethiopia. (c) Detailed map of the Chilga area showing the fossil localities, geological section (see Figure 2) and dated rock samples along the Guang and Hauga rivers. nated by the weathering-resistant teeth of larger of a marked change in lithology or sedimentary fea­ mammals, show little change during the time inter­ tures, whereas profile bases were delineated at the val represented at Chilga. The plant assemblages, highest occurrence of unaltered parent material. some of which occur in direct association with ver­ Field descriptions of paleosols (e.g., thickness, tebrate fossils, provide an excellent record of the color, type, and distribution of mottling; soil struc­ environment in which the animals lived and, with ture and mineralogy; size, morphology, and distri­ sedimentological data, provide a fine-scale assess­ bution of authigenic minerals) follow the methods ment of spatial and temporal environmental varia­ of Retallack (1988). Paleosol and lithologic colors tion. Below, we document and integrate were identified in the field with dry samples using sedimentological and paleontological data in order Munsell color charts (Munsell Color 1975). In order to better understand the landscape and climate at to avoid recent weathering, outcrop faces were dug Chilga. back minimally 60 cm to provide a fresh surface for describing and sampling the paleosols. Approxi­ METHODS mately 500 g of paleosol matrix was collected from each paleosol horizon and stored in either canvas Both sediments and plant fossils were sys­ or plastic bags. tematically collected for paleoenvironmental analy­ The mineralogical composition of sand-size sis. Paleosols were logged and described in the and coarser grains, as well as soil microfabrics, field following previously established methods were determined petrographically from doubly-pol- (Tabor et al. 2002; Tabor and Montañez 2004). ished thin sections following the methods of Moor- Pedogenic horizons within the Chilga